Does Math always work?

KCA_2

Hello all....
I've noticed that, from time to time, what I calculate shouldn't work, does..... not as much the opposite....

For instance.... I was on a job the other day and the system had 4psi in it at the boiler.... I always carry my own 3" diameter 30psi pressure gauge so I know it works..... the job was a two story home and the guy says that he has heat on the upper floor.... 16 plus feet above the boiler..... he has baseboard heat..... sorry.... I don't get it.... do any of you?

:-)) Ken

vaporvac

Maybe he had steam masquerading as HW?

Paul48

Sure.....Pressure is not needed for circulation. You set the auto-fill regulator to a certain pressure so the system will remain filled to the highest point of radiation. Bearing in mind, you will always have the weight of the water in the system registering a pressure, even if none is added.

SWEI

If the system is tight, you can run it with negative pressure in the top of the system. Should you? Probably not.

Alan (California Radiant) Forbes

Math always works; so does physics, no? You've just got to figure out how. That's where the fun is although it might not seem so at the moment.

GW

what is that special number, 32' or something like that? Beyond (higher) than that you need pressure

ChrisJ

Isn't that 15 PSI only to push air out of the system?
Once the system is full of water, wouldn't 0 PSIG static work fine?

jumper

If pressure is too low at bottom then there isn't water high enough. Rads above feet of water pressure are empty.Will a circulator force water through empty rads? Certainly if piping is overhead distribution.At least I think so.

Gordy

System psi is to insure water gets to the highest point in the system, and stays there. As Kurt said in theory you could Run the system with 0 psi or negative psi in a tight system. The circulator differential is what moves the water as Hatty noted.

Example if I fill the system to 15 psi then isolate the x tank, and relieve pressure in system to 0 psi the circulator will still move water with its pressure differential. How ever some interesting air issues will arise as air comes out of solution when water is heated. Along with the fact system psi will climb because x tank is isolated.

GW

No that's not what I was talking about

Just search Barometric Loop, you can send water 33.9 feet up (ok maybe it's 33.8) with no excess pressure ( of course the water will have its column weight).

Gordy

That's for open systems to prevent back siphoning. The premise is water will not rise above 33.9' at,sea level.

Not sure how it applies here.

SWEI

Hatterasguy said:

The difference could be 3-4 psi on a typical system. Add that to your observed 4 psi and you have the necessary pressure for the second floor.

When the circulator is running. I'd still rather see at least a couple of PSI at the highest point with everything off. 16 feet of head comes to 7 PSI. I would set the static pressure at 12 - 15 PSI. What's the advantage of leaving it low?

GW

I'm just blabbering, I'm confused about the topic in general. I installed a steam boiler 20 something years ago with a second floor hot water loop, simple flow checks. Zero pressure on that system, in the water stayed there.

Paul Pollets

Some of this depends on where the pump is located and if the system is"pumping away" from the connection of the expansion tank and fill.

KCA_2

Thanks for all of your comments.....
If the system were full with a 16' head I'd have about 7.5lbs static..... I only had 4lbs..... that meant to me that the system wasn't full..... the circulator isn't a pump.... it wouldn't heat the 2nd floor..... am I thinking wrong?

GW

Then let's talk vacuum. Just like when you hold your finger on the straw, that column is basically weightless.

Gordy

KCA said:

Thanks for all of your comments.....
If the system were full with a 16' head I'd have about 7.5lbs static..... I only had 4lbs..... that meant to me that the system wasn't full..... the circulator isn't a pump.... it wouldn't heat the 2nd floor..... am I thinking wrong?

With a circulator the water is moving like a Ferris wheel. The weight of the water going up is equal to the weight of the water going down. The pressure differential at the circ overcomes the head losses of the piping, valves, elbows, HX, etc.

Technically you would only need 6.9 psi to get to the top of the 16' system. .43 psi for every foot from the psi gauge to the top of the emitter.

Zman

KCA said:

Hello all....
I've noticed that, from time to time, what I calculate shouldn't work, does..... not as much the opposite....

For instance.... I was on a job the other day and the system had 4psi in it at the boiler.... I always carry my own 3" diameter 30psi pressure gauge so I know it works..... the job was a two story home and the guy says that he has heat on the upper floor.... 16 plus feet above the boiler..... he has baseboard heat..... sorry.... I don't get it.... do any of you?

:-)) Ken

</blockquote

Assuming that the system is not just 1/2 full and circ is not just sloshing water over the top, I would say that if a gage was installed on the top of the system, it would read -3psi. It just like holding water in a straw.

Paul48

"Then let's talk vacuum. Just like when you hold your finger on the straw, that column is basically weightless"

In a closed loop, that is not possible.

Gordy

Ken did you verify the top emitters, or just took the homeowners word for it that they heated fine. Maybe they heated, but not fine.

System psi does more than get the water to the top. Higher pressure also helps keep air in solution to be moved along to the air removal device.

This is one of the reasons pumping away from the point of no pressure change has benefits.

Jason_13

Also remember the lower pressure dependent on water temp could flash to steam.

GW

Negative pressure will lower the flash point below 212, presumably we aren't talking about flash becasue the loop is moving water.

Paul, you're sure the barometric loop principal isn't applying here? I'm not trying to say we have a farm out back and we are trying to keep the pesticides from getting into the drinking water. I'm saying if the pressure goes low, the water is still hanging in the loop.

Paul48

The theory of preventing back-siphonage based on the principle that the highest water can be siphoned up into a water line at sea level is 33.9 feet? How would that apply? Are you saying that in the absence of an applied pressure, a vacuum always exists, in a closed-loop system?
In order to prove this, you open a valve on the bottom of the system......And, Voila...the water stays in the pipe. This is faulty logic. You create the conditions for this to happen by opening the valve. It's like noting the ground being wet, after it's just rained, and then stating that every time the ground is wet, it's just rained.

Jody_S

I am late in this discussion, but Ken are you sure about the accuracy of your gage? (it's the engineer in me)

Jody from Caleffi.

Jody_S

Gauge that is. (Doing too many things at one, I guess I must pay attention)

Jody

hot_rod

Drainback solar and outdoor unpressurized boilers work without the need for any pressure. And many wood boilers pump with unpressurized buffer tanks.

Adding pressure to a drainback solar assures some pump head and suppresses the boiling point, eliminates flashing in the collector, and is highly recommended.

The circ pump wants some NPSH, usually around 4 psi at 190F

That 32' is the common number when considering maintain ing a siphon for drawbacks, lower at altitude of course. We have seen some solar drawback struggle to maintain a siphon at 27" in mountain installs.

Adding pressure helps evacuate air also as it squeezes the bubbles smaller so they wash bucket the purger.

I would guess his system with 4 psi has some noise issues?

Tom_133

This is one of those threads I read and realize I still have a lot to learn. Thanks all, I will be doing some reading.

GW

I half heartedly applied my so so understanding of the basic principles of science to the thread. So where dId we end up? The system with 4 psig can or can't have a functioning second floor? Thrird floor?

Gordy

Depends

In the original post it is functioning. For me it's the details of readings, and what constitutes functioning.

I think we have concluded in theory it can.

SWEI

In a sealed system, gauge pressure need only be sufficient to control the boiling point, which applies to both water and steam systems.

Differential pressure is what matters in the real world. Water tight and vacuum tight are somewhat different standards, especially when applied to valve packings, pump seals, and the like.

hot_rod

In Idronics 10 we go over the physics of a non, or low pressure open hydronic system. Most of the OWF only run a few psi, whatever the highest point in the OWF, the overflow tube, dictates then static pressure, most are about 6 feet tall so 6 X .433= 2.6 psi.
This also explains why the circ pumps on open OWF cavitate to death. At 190F a typical small wet rotor wants around 4.6 min. psi. High head circs maybe more?

Taco offers a 3 piece OWF circulator, this at least keeps the motor out of the fluid if or when it flashes.

The OWF pump behaves more like a drainback solar pump, lifting the water to the highest point. But in a drainback solar you can add pressure to suppress the fluid flashing in hot collectors, and provide some additional pressure at the circ.

Paul48

After all the posts here, people are still unsure of the answer to the original question. Having to read an article about drainback solar, and then figure out how it applies, or if it applies to a sealed system, certainly doesn't simplify anything. Dan had a posting a while ago, about being able to explain something so that anyone could understand it. If you were teaching a 3 year old that it was socially improper to pick his nose, would you start by reading him a manual on the proper way to trim his finger nails? No disrespect intended folks, just an observation.

GW

Many years ago I married into a batch of smart folks. My wife and I are like Donnie and Marie, I explain mechanical things to her, she explains basic grammar rules to me, what a deal.

I was on the phone the other night asking my pops-in-law about my confusion with the 33.8'. Now i have a firmer grip on the matter, just because the gauge says 4 doesn't mean it can't operate (it certainly can).

Thanks all, I enjoy the learning process

hot_rod

The OP indicated he is a serviceman with tools and experience so he has some basic understanding we assume. It's tough to know how basic the advice or suggestions need to be for every question. I think all responses added good explanations